1. Field of the Invention
The present invention relates to a method and a device for driving a plasma display panel (abbreviated as "PDP"), used as a flat display easy to realize a large display area, for example as a display for a personal computer and a work station, and a wall television receiver, and more specifically to a method and a device for driving an AC drive type plasma display panel (abbreviated as "AC-PDP").
2. Description of Related Art
In general, the PDP is divided into a DC type and an AC type, on the basis of a difference in a driving method. In the DC type, electrodes are exposed to a discharge gas, and a discharge occurs only a period in which a voltage is applied. In the AC type, electrodes are covered with a dielectric film and a discharge occurs without exposing the electrodes to a discharge gas. In the AC-PDP, furthermore, a discharge cell itself has a memory function because of an electric charge accumulating action of the dielectric film.
Referring to FIG. 1, there is shown a diagrammatic section view of a conventional AC-PDP. The shown AC-PDP includes a front substrate 10 and a back substrate 11 which are formed of a glass and which are fixed separately from each other by a predetermined distance to define a space therebetween.
On an inside surface of the front substrate 10, scan electrodes 12 and common electrodes 13 are formed with a predetermined interval, and covered with an insulating layer 15a, which is coated with a protection layer 16 formed of for example MgO, etc., in order to protect the insulating layer 15a from an electric discharge.
On an inside surface of the back substrate 11, data electrodes 19 are formed to extend in a direction orthogonal to the scan electrodes 12 and the common electrodes 13. The data electrodes 19 are covered with an insulating layer 15b, which is coated with a phosphor 18 in order to convert ultraviolet generated by the electric discharge, into a visible light.
Between the insulating layer 15a of the front substrate 10 and the insulating layer 15b of the back substrate 11, a partition wall 17 is formed to ensure a discharge space 20 between the front substrate 10 and the back substrate 11 and also to define each pixel. A discharge gas composed of a mixed gas of He, Ne, Xe, and others is sealed in the discharge space.
Referring to FIG. 2, there is shown a diagrammatic plan view of the scan electrodes 12, the common electrodes 13 and the data electrodes 19 shown in FIG. 1.
In FIG. 2, "m" scan electrodes Si (i=1, 2, . . . , m) are formed in a row direction, and "n" data electrodes Dj (j=1, 2 . . . , n) are formed in a column direction, so that one pixel is formed at each of intersections between the scan electrodes Si and the data electrodes Dj. Common electrodes Ci are formed in the row direction so that each of the common electrodes Ci is paired with a corresponding one of the scan electrodes Si. In the shown example, the common electrodes Ci and the scan electrodes Si are located in parallel. If the phosphor 18 shown in FIG. 1 is divided into three colors of red, green and blue in units of pixel, a color display PDP can be obtained.
Now, a method for driving the conventional PDP will be described with reference to FIG. 3, which is a timing chart illustrating various driving voltages applied to respective electrodes of the AC-PDP shown in FIGS. 1 and 2.
First, an erase pulse 21 is applied to all the scan electrodes S1 to Sm, to erase light emitting pixels which had emitted light until the erase pulse 21 is applied. As a result, all the pixels are put into an erased condition.
Next, a pre-discharge pulse 22 is applied to all the common electrodes C1 to Cm, to forcibly cause an electric discharge in all the pixels, and then, a pre-discharge erase pulse 23 is applied to all the scan electrodes S1 to Sm, to erase the pre-discharge of all the pixels. This pre-discharge facilitates a writing discharge which will be carried out later. Therefore, since this pre-discharge for facilitating the writing discharge has to be certainly caused in all the pixels, the pre-discharge pulse 22 is generally set to have a sufficiently high voltage and a sufficiently long pulse width.
After the erase of the pre-discharge, a scan pulse 24 is applied to the scan electrodes S1 to Sm at timings shifted from one another, respectively, and on the other hand, data pulses 27 are applied to the data electrodes D1 to Dn, respectively, in time with application of the scan pulse 24. In FIG. 3, a slash given in the data pulses 27 indicates that presence/non-presence of the data pulse has been determined in accordance with presence/non-presence of a display data. In the pixel in which the data pulse 27 is applied when the scan pulse 24 is applied, a writing discharge is generated in the discharge space 20 between the scan electrode 12 and the data electrode 19, but in the pixel in which the data pulse 27 is not applied, no writing discharge is generated.
In the pixel in which the writing discharge has been generated, namely, in the pixel which the display data is "ON", a positive electric charge, called a "wall charge", is accumulated in the insulating layer 15a on the scan electrode 12. At this time, a negative wall charge is accumulated in the insulating layer 15b on the scan electrode 12.
Thereafter, with superposition of the positive potential created by the positive wall charge established in the insulating layer 15a of the scan electrode 12 and a first sustain pulse 25 which is negative and which is applied to the common electrodes 13, a first sustain discharge occurs. If the first sustain discharge occurs, a positive wall charge is accumulated in the insulating layer 15a on the common electrode 13, and at this time, a negative wall charge is accumulated in the insulating layer 15a on the scan electrode 12. Succeedingly, a second sustain pulse 26 applied to the scan electrode 12 is superposed on a potential difference given by these wall charges, a second sustain discharge occurs. Thus, the sustain discharge continues by superposition of the potential difference given by the wall charges formed by an (x)th sustain discharge with an (x+1)th sustain pulse. The quantity of emitted light is controlled by the number of sustain discharges.
By previously determining the voltage of the sustain pulses 25 and 26 at such a degree that the discharge never occurs with the sustain pulse alone, in the pixel in which the writing discharge had not occurred, since the potential given by the wall electric charge does not exist before application of the first sustain pulse 25, the first sustain discharge does not occur although the first sustain pulse 25 is applied, and thereafter, no sustain discharge occurs.
The display contrast ratio is a value by diving the luminance when the display data is "ON", by the luminance when the display data is "OFF". In the color PDP, because of the above mentioned driving operation, in the pixel corresponding to the display data of "OFF", there occurs no light emission which is caused by the writing discharge and the sustain discharge following the writing discharge, but there exists a light emission caused by the pre-discharge. On the other hand, in the pixel corresponding to the display data of "ON", there occurs not only the light emission caused by the writing discharge and the sustain discharge following the writing discharge, but also a light emission caused by the pre-discharge. Accordingly, the display contrast ratio in the color PDP is a value obtained by dividing a total of the luminance of the light emission caused by the pre-discharge, the luminance of the light emission caused by the writing discharge and the luminance of the light emission caused by the sustain discharge following the writing discharge, by the luminance of the light emission caused by the pre-discharge. Therefore, the display contrast ratio in the color PDP can be elevated by making large the luminance of the light emission caused by the writing discharge and the luminance of the light emission caused by the sustain discharge, and/or by making the luminance of the light emission caused by the pre-discharge small.
The erase pulse 21, the pre-discharge pulse 22, the pre-discharge erase pulse 23, the scan pulse, the sustain pulses 25 and 26, and the data pulses as mentioned above are conventionally constituted of a rectangular pulse having a rising time of not greater than 1 microsecond and a falling time of not greater than 1 microsecond.
FIG. 4A illustrates a driving rectangular pulse applied to the PDP shown in FIG. 1, and FIG. 4B illustrates a discharge current flowing when the driving rectangular pulse shown in FIG. 4A is applied.
This discharge current starts to flow with a delay of several hundred nanoseconds from application of the voltage pulse, and reaches a peak with a delay of several hundred nanoseconds from the moment the discharge current starts to flow, and the discharge current terminates after it continues several hundred nanoseconds from the peak. The time from the application of the pulse to the moment the discharge current starts to flow, the time from the moment the discharge current starts to flow to the moment the moment the discharge current reaches its peak, and the continuing time of the discharge current after the peak, are determined by the structure of the PDP, including the composition of the discharge gas, the composition and the thickness of the dielectric layer, the composition and the size of the electrodes, and the size of the discharge space. When the discharge is caused by the rectangular pulse shown in FIG. 4A, the higher the peak value "V" of the pulse is, and the larger the pulse width "t" is, the large the luminance of the emitted light becomes.
In the above mentioned AC-PDP, in order to stably generate the writing discharge, it is necessary to surely cause the pre-discharge. In the above mentioned conventional AC-PDP driving method, in order to elevate the sureness of the generation of the pre-discharge, the pre-discharge pulse was set to have a high voltage and a long pulse width. As a result, the luminance of the emitted light caused by the pre-discharge has become large, with the result that the display contrast ratio has become low.
Furthermore, in the conventional PDP, the luminance efficiency of the discharge is low, and therefore, the power consumption is large.